1. Field of the Invention
The present invention relates to an ink composition suitable for ink jet recording, and a fine particle of a polymer suitable for the ink composition for ink jet recording.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink are ejected and deposited onto a recording medium such as paper. The ink for the ink jet recording generally contains a water-soluble dye in water or a combination of water with an organic solvent. It is generally pointed out that an image obtained by the ink containing a water-soluble dye has poor waterfastness and lightfastness.
On the other hand, an ink prepared by dispersing a pigment in an aqueous medium has excellent waterfastness and lightfastness. In an image obtained by the ink containing a pigment as the colorant, however, the colorant is likely to be left near the surface of the recording medium. Therefore, unsatisfactory fixation of the colorant on the surface of the recording medium results in the formation of a print which, when the image is rubbed with, for example, a finger, causes the recording medium to be smeared with the pigment. Furthermore, when printed letters are marked with a highlight pen, this causes the printed image area to be smeared. Therefore, a print having a good touch and rubbing/scratch resistance cannot be provided. Addition of a resin to the ink composition has been proposed in order to improve the fixation of the colorant to the recording medium. This resin is considered to function as a binder to strongly fix the colorant onto the recording medium.
Conventional resin-containing ink compositions include, for example, an ink comprising a pigment and a resin emulsion dispersed in water (Japanese Patent Publication No. 1426/1987), a dispersion of a pigment in a water-insoluble, resin emulsion dispersion (Japanese Patent Laid-Open No. 157668/1980), an ink using an emulsion having a specific film-forming temperature (Japanese Patent Laid-Open No. 217088/1989), and an ink using a resin emulsion (Japanese Patent Laid-Open Nos. 60068/1991 and 18462/1992). Further, Japanese Patent Laid-Open Nos. 147859/1981 and 147860/1981 and Japanese Patent Publication No. 5703/1992 propose an aqueous dispersion type pigment ink using a polymeric dispersant and a water-soluble organic solvent.
Further, use of a core/shell resin particle comprising a core and a shell surrounding the core in an ink for ink jet recording has also been studied in the art. For example, Japanese Patent Laid-Open No. 299234/1991 proposes an ink comprising a fine particle of a polymer having a two-layer structure and having an average particle diameter of not more than 0.1 .mu.m. The particle comprising a core polymer prepared by homopolymerization or copolymerization of a (meth)acrylic ester compound, a vinyl ester compound, a styrene compound, or an olefin compound and a shell polymer comprising a fluoropolymer. Japanese Patent Laid-Open No. 259869/1996 proposes use of an organic fine particle comprising a core of a polymer, having a high degree of crosslinking, such as a styrene/divinylbenzene polymer, and a shell having a surface-modifying property.
Japanese Patent Laid-Open No. 278479/1995 proposes use of a fine particle prepared by polymerization of a monomer containing not less than 5% by weight of a crosslinking monomer having in one molecule two or more groups having a polymerizable double bond and not less than 2% by weight of a hydrophilic monomer having a hydrophilic group.
However, some of the ink compositions containing the above resins have a viscosity rendering the ink composition unsuitable for ink jet recording. In addition, a head for ink jet recording, has a nozzle plate which is treated so as to be rendered water-repellent, permitting ink droplets to be easily ejected through nozzles. Some of the resin-containing ink compositions are likely to attach to the nozzle plate and to well wet the plate. This causes an ink droplet trajectory directionality problem or a failure of the nozzle to eject the ink. Further, due to the water-soluble property of the resin, the printed image has often poor waterfastness. Furthermore, evaporation of water in the ink increases the viscosity of the ink, leading to clogging of nozzles.
In addition, the fine particle of the polymer disclosed in Japanese Patent Laid-Open No. 278479/1995 contains not less than 5% by weight of a crosslinkable monomer, causing a high degree of crosslinking. This makes it impossible to provide a satisfactory film-forming property, resulting in unsatisfactory fixation of a printed image onto the recording medium. Therefore, the printed image has poor rubbing/scratch resistance.